1. Field of the Invention
The present invention pertains to the identification of subscribers in a party line telephone system and, more particularly, to an identifier circuit for placing a selective ground mark on a party line representative of a calling subscriber.
2. Discussion of the Prior Art
As an alternative to verbally requesting a telephone operator to place a long distance call, most telephone subscribers in the United States and Canada can place such calls via Direct Distance Dailing (DDD). For obvious reasons, a subscriber placing a DDD call must be identified, and such identity must be associated with a record of the time the call was placed, the duration of the call and the telephone number called to be later associated with the calling subscriber's account for billing purposes. In the telephone industry, a subscriber's telephone number is usually his account number. A telephone number is assigned to a particular outlet/inlet (appearance) of a switching matrix at a central office. Therefore, for billing purposes, a DDD call need only be associated with a telephone number irrespective of which telephone instrument (extension) placed the call. In most cases, one pair of wires extends from the central office to the subscriber's premises (home or business) where one or more telephone instruments are connected in parallel to that pair. Even if the telephone instruments are located in different buildings and such buildings are connected via separate pairs to the central office electrically, such telephone extensions will have the same number.
There are two basic methods of identifying a calling subscriber. Operator Number Identification (ONI) requires the intervention of an operator at the completion of dialing of the called number. The operator asks the caller his telephone number and records this number on a record associated with the details of the call (time and called number). Time of answer, if any, and time of disconnect or elapsed connect time are later recorded. Automatic Number Identification (ANI) works the same way with the exception that the operator is replaced by electronic equipment at a central office that determines through which matrix appearance a call was placed and records this information as described above, usually on magnetic or paper tape. Obvious disadvantages of ONI include high recurring cost of operators, billing errors due to operator error, and billing errors due to intentional or accidental transmission by the subscriber of an incorrect number.
So long as only one subscriber is connected to a matrix appearance, automatic number identification is confined to determining which appearance on the matrix placed the call in question. Frequently, however, in order to save switching apparatus and wire, two or more subscribers are connected to the same appearance and cable pair. Thus, we have the party line which allows neighboring premises to be connected in parallel to a single pair of leads connected to the central office, often many miles distant. Such arrangements are common in suburban and rural areas where the high cost of outside cable and central office equipment must be shared by multiple subscribers in order to keep rates down, and/or because the telephone utility lacks the time or resources to install sufficient lead pairs and equipment to provide each subscriber with a private line. Party lines complicate the ANI task by requiring that the equipment in the central office identify not only the matrix appearance but which of several subscribers connected at a distance via one pair of leads to this appearance actually placed the call.
Two common party line identification schemes are called circle digit and ground mark. Circle digit is so named because, in addition to other numbers, the calling subscriber must dial a digit printed in a circle on the number plate attached to the dial of his telephone instrument, which digit is unique to each subscriber on the party line. This method can uniquely identify up to ten parties on a line; however, like ONI, is subject to erroneous billing due to accidental or intentional transmission of an incorrect digit.
Ground mark type identification schemes involve placing a resistance ground on one of the pair of leads, commonly referred to as tip and ring, of the party line while the telephone instrument is off hook. This ground is usually derived from a 2650 ohm winding in the bell. Thus, ringing and identification schemes can complement each other and provide two-party selective ringing as well as party identification by connecting the bells in one party's telephone instruments from ground through a capacitor to the ring lead and in the other party's telephone instruments from ground through a capacitor to the tip lead while the telephone instrument is on hook and directly to both sides of the line when the telephone instrument is off hook. This scheme can be expanded to identify four parties by connecting the bells as above for parties one and two, and through diodes for parties three and four.
The major problem with ground mark identification is longitudinal imbalance, it being necessary, due to the presence of sometimes very high induced AC voltages from commercial power distribution, that telephone speech circuits be AC balanced with respect to ground. In a perfectly balanced circuit, power noise will be induced equally into both leads of the line. Therefore, common mode noise signals will cancel each other, and subscribers will hear no noise. Any imbalance, such as a ringer to ground will cause the common mode rejection quality (called longitudinal balance) to degrade. Unfortunately, the circuits which are most likely to be party lines (long rural loops) are also subject to the greatest common mode voltage since telephone lines are normally attached to poles or buried along power distribution rights of way.
A secondary disadvantage of ground mark identification in the requirement that all telephone instruments on the subscribers premises be wired in the appropriate manner for that party. Thus, if a subscriber connect his own phone to the line, billing errors are likely.
Since most non-circle digit ANI systems use some ground mark method for party identification, there is a need to apply the ground mark only during the identification period so as not to unbalance the line during talking. A device for providing such function should be designed for installation outside the premises so that one device can serve all phones at that subscriber's location, and so that the installer does not have to gain access to the inside of the subscriber's premises. Several prior art devices have been developed with the above considerations in mind; however, such devices have been more successful at serving all phones at a location, such as by a simple series connection which allows detection of current flow due to one or more phones on the premises being off hook, than at applying a ground mark during dialing. A few devices use a method other than ground marking, which has the disadvantage of making them applicable only to special automatic number identifying equipment at the central office. Other devices leave the ground mark in during part or all of the conversation, for example, until the polarity reverses. Such devices are useful only in step exchanges on completed outgoing local calls. Incoming calls, operator calls, and perhaps outgoing long distance calls do not reverse the polarity of the tip and ring leads and, therefore, allow the ground mark to remain on the line having thus the disadvantage of causing an imbalance. Another prior art device serves only for two party identification and interferes with dial pulsing by applying the ground mark during each open period (pulse).
U.S. Pat. Nos. 2,794,859 to Abbott et al., 2,820,100 to Abbott, 2,901,544 to Collins, 3,005,052 to Powers, 3,760,112 to Bush, 3,927,270 to Davis and 4,001,512 to Procter et al. are exemplary of the above discussed prior art attempts to provide devices for two or four party identification in party line telephone systems.